Chemical transmission in the central nervous system occurs at synapses, the site of interaction between neurons. Release of neurotransmitter activates receptors on the postsynaptic cell, perpetuating signals from one neuron to the next, and control of the number and activity of receptors is crucial for brain function. Modification of synapses is necessary for development and learning and memory, and synaptic dysfunction is associated with many neurological disorders. Synaptic modification involves structural changes and this function is served by the postsynaptic density (PSD), a macromolecular protein machine that resides under the postsynaptic plasma membrane. The PSD regulates the efficiency of synaptic transmission by stabilizing neurotransmitter receptors in the synaptic membrane and functionally organizing signaling molecules within the postsynaptic compartment. Data suggests that synaptic activity results in changes in the protein composition of the PSD and it is hypothesized that these changes lead to long-term structural modifications that explain enduring and stable alterations in synaptic function. However, the extent of dynamic remodeling and the mechanisms responsible are not fully understood. It has been proposed that the ubiquitin proteasome system (UPS), a regulated system that targets proteins for degradation, is, in part, responsible for structural modifications of the PSD. Synaptic activity wa also shown to induce proteasomal recruitment into the postsynaptic compartment and that this requires activity-dependent sequestration of CaMKII. It has also been shown that proteasome activity is responsible for changes in the protein composition of the PSD in response to synaptic activity. These findings support the hypothesis motivating this proposal: In response to synaptic activation, there is structural remodeling of the PSD mediated through proteasomal recruitment and targeted protein degradation of key scaffold molecules. The goal is to investigate the role of the UPS in remodeling the PSD using state-of-the-art electron microscopic imaging techniques. The first objective will be to determine whether synaptic activity results in structural changes to PSDs through proteasomal degradation of PSD scaffold proteins. Electron cryo-tomography (ECT) and immunogold labeling will be employed to examine the 3D structure of isolated PSDs and to identify scaffold molecules targeted by the UPS. This aim will focus on identifying specific PSD scaffold molecules targeted by the proteasome and the resulting changes to the PSD structure with and without inhibition of proteasome activity. The second objective will be to determine whether proteasomal recruitment into the PSD complex requires prior CaMKII translocation in response to synaptic activity. This aim will again employ ECT and immunogold labeling to examine where the proteasome is incorporated into the PSD structure, what the spatial organization of the proteasome is within the PSD and whether these events directly require CaMKII recruitment. Together these aims will provide insight to a mechanism for activity-induced changes to the PSD structure and ultimately synaptic function.